This invention relates to fuses, to electrical fuses in general and more particularly to an improved link for an electrical fuse. Essentially, there are many fuses which exist on the market where a manufacturer will deposit a globule or a low melting metal on a fuse link. The purpose for this is to reduce the temperature at which the link will melt whereby the lower melting metal provides a hot spot on the link which becomes the point at which the fuse will open under overload. For example, many links are fabricated from copper which is a good conductor and has a relatively high melting point. In providing a fuse with a lower melting point, manufacturer may deposit a globule or a rivet fabricated from selenium, germanium, indium or tin. For example, copper melts at 1600.degree. F. while tin has a melting point of about 1300.degree. F. A composite link fabricated from copper with a tin globule or rivet will have a reduced melting point between 400.degree. F. to 600.degree. F. depending upon the concentration of the low melting temperature metal on the link.
This technique, as indicated, has been widely employed in the prior art. This is known as the M effect. There is a problem with the M effect at certain overload conditions. What occurs is that as the fuse is being operated under overload conditions, the copper heats up and the tin tends to move or creep towards the hot spots of the fuse link which are normally associated with reduced cross sectional areas of the link. Under normal circumstances, this would disolve the copper in the reduced cross sections causing them to get extremely hot aand completely melt. This causes the fuse to open which is the desired result.
It has been found that this is not always the case. Under very small overloads, the tin will migrate to the reduced cross sections as described above, but it will migrate in such small amounts that it will not affect the operation of the reduced cross section that the tin has migrated to. The tin will then continue to migrate down the link until it has reached a relatively uniform concentration down the entire link. The end result of this is that the fuse will never open under overload conditions and this will adversely effect the short circuit operation. The migration of the low melting point metal completely changes the characteristics of the fuse.
In this manner, the hot spot point in the fuse link which as indicated occurs at the reduced cross sectional areas of the link draws the low melting point metal down the link further causing the same to absorb into the copper. This completely changes the characteristics of the fuse, and hence the area in which the low melting point metal was placed does not have enough of the metal due to the migration of the same, and therefore the fuses will not operate as designed. Essentially, it is an object of this invention to provide a means for retaining the low melting point material in the desired area of the copper fuse link and hence to prevent migration of the metal towards the ends of the fuse.
In order to do so, the area which is designated by the low melting point metal is encapsulated by means of a non-conductive high temperature plastic enclosure. The encapsulated section completely defines a predetermined hot spot for the fuse and prevents the low melting point metal which is deposited on the link from moving down the link to thereby alter the characteristics of the same.
There are many patents in existence which show a fuse having an encased element for various purposes. For example, U.S. Pat. No. 2,543,245 entitled Fuse Construction issued on Feb. 27, 1951 to G. F. Laing. This patent shows a fuse having an eutectic alloy element which is separately encased between two partition walls with each element end connected to a suitable link member.
U.S. Pat. No. 2,561,464 entitled Time Lage Fuses issued on July 24, 1951 to M. Cremer discloses a fuse which has an alloy in the form of a spherical mass positioned on a reduced cross sectional area of the element. This area is further isolated by means of partition walls.
Another patent, U.S. Pat. No. 2,577,531, entitled Fuse Construction issued on Dec. 4, 1951 by G. F. Laing shows a dual element fuse in which the links have a higher melting point than the central portion which is also an eutectic alloy and is similar in construction to the fuses shown in U.S. Pat. No. 2,577,531.
Other patents such as U.S. Pat. Nos. 3,701,069 and 3,721,935 show various fuse constructions which consist of a central element separated by an insulating casing from a fuse link. In any event, all these fuses are composite fuses. That is to say, the fuses include dual elements or can be represented by dual elements which are composite materials secured to central link arrangement. None of the prior art references depicted show a fuse construction which prevents the above noted problem in an efficient and reliable manner.